Student Project Abstracts 2005 - Pluto - University of Washington

ELECTROLUMINESCENT PROPERTIES OF ORGANIC LIGHT-EMITTING DIODES (OLEDS) WITH 2,5-BIS(9,9-DIMETHYLFLUOREN-2-YL)-1,1,3,4-TETRAPHENYLSILOLEcleaned with acetone and ethanol. The silicon oxide slideswere then placed in a microwave vacuum chamber where theywere oxygen-plasma treated. The silicon oxide served as aninsulation layer to limit area of emission of each OLED device.The order of deposition on the silicon oxide slides wasorganic layers, lithium fluoride (LiF), and aluminum. The organicand inorganic layers and their thicknesses are listed asfollows: TPD (40 nm), Alq 3(40 nm), LiF (1 nm), and Al (300nm). The thickness of DTS varied. The LiF was used to helpthe injection of electrons by the aluminum cathode. Four combinationsof deposition of the organic layers were made correspondingto the four different types of devices: ETL, HTL,EML and single layer (SIL). ETL, HTL EML devices had aDTS thickness of 40 nm. SIL device had a DTS thickness of120 nm.Tests An Ocean Optics fiber spectrometer was used tomeasure the electroluminescent (EL) spectra of each device[ 4 ]. Figure 2 shows the normalized EL spectra of each type ofdevice. To measure the EL characteristics of the devices eachdevice was placed in an electrical contact box fixated directlyin front of a photodetector. The electrical contact box andphotodetector where then covered with a black box to blockoutside light. The applied voltage was varied using a Keithley2400 sourcemeter. The current through each device and thelight emitted by the device was measured. All tests were automatedusing Labview software. The luminance and the externalquantum efficiency (EQE) was then calculated by normalizingthe measurement of the photodiode to the photopicresponse of the human eye and the sensitivity response of thephotodiode. Figures 3 through 6 show the mean luminescence,EQE, and current density curves for each device structure as afunction of applied voltage [5].The luminescent lifetime of each device was measured ata constant current density of 50 mA/cm 2 . The voltage neededto keep that current was automatically adjusted by the sourcemeter. Figures 7 and 8 show the lifetime measurements of theHTL, EML, and SIL devices.RESULTSAll devices emit bright green light. The ETL, HTL, and EMLdevices emit light at a wavelength of 535 nm, while the SIL deviceemits light at a wavelength of 540 nm.Table 1 shows the applied voltage, luminance, EQE, and currentdensity and their respective errors at the maximum EQE.For a constant current density of 50 mA/cm 2 the HTL, EML,and SIL had half lives of 840, 1100, and 4000 s, respectively.Lifetime measurements of the ETL devices could not be obtaineddue to a significant amount of leakage as can be seen in figure 4.Figure 2. Normalized EL spectra of ETL, HTL, EML, and SIL devices.Figure 3. Averaged luminance and EQE of ETL andHTL devices as a function of applied voltage.Figure 4. Averaged current density of ETL and HTLdevices as a function of applied voltage.Device Vappl. Luminance (±Er) EQE(±Er) Current Density(±Er)(V) (cd/m 2 ) (%) (mA/cm 2 )ETL 8.0 1300(±100) 0.3(±0.2) 115(±5)HTL 10.8 850(±150) 0.3(±0.1) 100(±50)EML 9.4 1550(±150) 1.0(±0.1) 45(±5)SIL 13.1 800(±200) 0.3(±0.1) 85(±5)Table 1. Maximum EQE results.116 CMDITR Review of Undergraduate Research Vol. 2 No. 1 Summer 2005